1. Field of the Invention
The present invention relates to a method for super calendering the surface of magnetic recording elements such as video tapes, memory tapes, audio tapes and the like, and to a super calendering roll assembly for the same.
2. Description of the Prior Art
The methods described below are known for super calendering the surface of magnetic layers of magnetic recording elements:
(1) A method which comprises improving either the properties of dispersing agents or dispersing means in the preparation of a magnetic coating liquid to form magnetic layers of less roughness on the surface thereof immediately after coating, i.e., since the surface is soft immediately after coating, while it is soft the surface is smoothened with a smoothing roll;
(2) A method which comprises, after coating and drying, abrasion-polishing the magnetic layer by moving the magnetic layer against a similar magnetic layer at a high relative rate, thereby super calendering the surfaces of the magnetic layers;
(3) A method which comprises abrading the magnetic layers by means of animal hair or a plastic or metallic brush to polish the same; and
(4) A method for forming the magnetic layers by means of a press roll called a super calender.
The four methods mentioned above, however, have the disadvantages indicated below:
In method (1), the electromagnetic conversion characteristics, mainly sensitivity and the S/N ratio, are not satisfactory.
In method (2), drop out increases due to scrapped powders generated from the magnetic layers during polishing; the method is not useful in a practical sense.
In method (3), the surface calendering required for high density recording is impossible.
In method (4), rolls made of crude cotton, absorbent cotton, asbestoes, pulp, cotton, wood and the like, such as cotton rolls, Filmat (compressed absorbent cotton) rolls, asbesto rolls and wood paper based rolls have been employed as elastic rolls. However, a sufficient super calendering effect is not obtained in these elastic rolls, i.e., there is too much noise (chromanoise) when color signals are recorded.
The disadvantages above are believed to be due to an insufficient super calendering effect on the magnetic layers and are caused by substantial unevenness (roughness) of the surfaces of the magnetic layers.
The present invention is concerned with an improvement of the surface properties of magnetic recording layers using a specific super calendering roll assembly to thereby decrease the chromanoise in video signals and the S/N ratio in audio signals.
The characteristics described above are improved by a decrease in envelope deviation. This will be better understood by referring to the drawings.
In the wave shape of a video output, as is shown in FIG. 1 (the ideal wave shape of the output), it is desired that the maximum level of the output reproduced be constant at all times, if the output level upon recording is constant. However, the output actually varies because of uneven contact of a magnetic head with a magnetic tape and other reasons, as is shown in FIG. 2 (the actual wave shape of the output).
While there is no art recognized technique to determine deviations in the output level, the ratio of the deviation of the output from the maximum output level is designated the envelope deviation in the present invention in order to quantitatively evaluate the deviation.
In the present invention, the evaluation of the envelope deviation is made in accordance with the following equation: EQU Vo/Vs .times. 100 = Envelope Deviation (%) (1)
wherein Vs: the 1/2 width of the maximum output of a carrier signal; and PA1 Vo: the output deviation width of the carrier signal. PA1 (a) Due to the large nip pressure on super calendering, the load which is placed on the roll is large so that the roll cannot tolerate the pressure which causes a noise called skewing. PA1 (b) Due to autogenously generated roll heat in super calendering, the center of the roll is expanded by thermal expansion to a drum shape as is shown in FIG. 4 (b), and, as a result, the pressure on the roll becomes uneven, which results in uneven thicknesses of the magnetic tape prepared. PA1 (c) If the thickness of the roll is increased, the thermal expansion becomes larger; if it is decreased, cracking or peeling-off occurs. PA1 (d) Due to uneven temperature distribution in the roll, uneven hardness results in the roll, and, as a result, the pressure becomes non-uniform to cause uneven thicknesses in the magnetic tape produced. PA1 (1) high hardness and must not be destroyed even under high linear pressures; PA1 (2) excellent abrasion resistance; PA1 (3) low electrical charging and must not absorb dust; PA1 (4) excellent surface properties; and PA1 (5) hardly undergoes changes in shape or soften at high temperatures. PA1 (6) the roll must have a structure from which thermal expansion due to autogenous heat of the elastic part of the roll is removed to thereby prevent distortion of the roll in shape which results in non-uniform pressure thereon; PA1 (7) to prevent the noise called skewing, the roll should be designed so as to not peel off and, at the same time, to avoid breakage due to cracking; PA1 (8) distortion of the shape of the roll, local increases in temperature and uneven hardness of the roll should be prevented; and PA1 (9) durability should be improved.
By this evaluation, the envelope deviation with conventional tapes is above 15%, while the envelope deviation is preferred to be below 13% for practical use. In the present invention, the envelope deviation is less than 10% at its worst.
As is shown in FIG. 3, the principle of super calendering is to press magnetic recording elements with a high linear pressure between metal rolls of excellent surface smoothness (M.sub.1, M.sub.2 and M.sub.3) and elastic rolls (E.sub.1 and E.sub.2). Super calendering of the surfaces of magnetic recording elements is due to the pressure between these rolls. In FIG. 3, A designates a guide roll and B designates a winding roll. The number of metal rolls (M.sub.1, M.sub.2, M.sub.3, M.sub.4,) and elastic rolls (E.sub.1, E.sub.2, E.sub.3,) may optionally be increased or decreased.
According to conventional methods for preparing such elastic rolls, there are known a method of merely casting a roll part having inserted therein a core and a method for mechanically molding resin into a pipe shape with the subsequent mechanical insertion thereof around a core or the subsequent insertion thereof around a core under heating.
The super calendering rolls prepared in these manners have a shape as is shown in FIG. 4 (a). These rolls are, however, disadvantageous at the following points:
In FIG. 4, numeral 1 designates an elastic part, numeral 2 designates a metal core of a roll and numeral 3 designates thermally expanded areas.
In order to eliminate the disadvantages described above, the materials of elastic rolls must meet the following criteria:
The following conditions are required in an elastic roll to obtain further improved super calendering effects, particularly operation for long periods of times.
Elastic rolls comprising urethane rubbers which are improved over conventional fiber rolls are known elastic rolls having the above characteristics (see Japanese Patent Application (OPI) 104611/74). However, these rolls still have insufficient mechanical strength and are practically disadvantageous.
In order to specifically meet requirements (6) to (9) above, an elastic roll is preferred to have a structure as is shown in FIGS. 5 to 10.